1. Field of the Invention
The invention relates generally to the field of calibration of thermal imaging devices. More specifically, the invention relates to a device for the in-situ calibration of an uncooled focal plane array imager, for instance, the calibration of a thermal imaging camera used in a small unmanned aerial vehicle or unmanned aerial system (“UAV” and “UAS” respectively) during flight.
2. Background of the Invention
Thermal imaging cameras (including UAV airborne thermal cameras) typically require single-point, pixel non-uniformity correction calibration against a uniform temperature scene to produce smooth, strip-free, ghost-free images. This calibration step is generally performed with the imager at a predetermined calibration temperature in, for instance, a temperature controlled calibration chamber. Nonetheless, in actual use and as the temperature of the imager changes while in operation (e.g., cools down due to air flow during flight or heats up due to heat dissipated from surrounding electronics), the imager will tend to drift out of calibration with resultant image-quality deterioration. Regular calibration of the imager during operation resolves the temperature-change related image-quality problems but in order to calibrate the imager in-flight, prior art cameras use a mechanical shutter element to provide a thermally uniform surface.
In prior art devices, an imager is calibrated to adjust for thermal drift and non-uniformity by closing a mechanical shutter whereby the imager is recalibrated by viewing a thermally uniform shutter surface and making electronic uniformity adjustments to the individual pixels on the focal plane array. In other words, the imager is calibrated in the field by making adjustments to the output of the individual pixels across the array while viewing a thermally uniform surface so that there is a uniform output when the imager sees a thermally uniform surface.
Unfortunately, prior art mechanical shutter elements have proven to be unreliable, relatively heavy and complex and thus are undesirable. As a result, it is common that UAS thermal imagers are frequently provided without a mechanical shutter element and therefore cannot be calibrated in flight.
A second problem the present invention solves is the risk of damaging an imager if directly viewing the sun. With no shutter (or with the shutter open when taking images or video), the imager by the nature of its design and materials, can be permanently damaged if directed toward the sun even for short periods of time. The current instructions for UAS operators are just “don't fly with the sun in your line of sight.” An unintentional maneuver or operator's error that allows the sun to appear in the camera's line of sight may cause camera damage. The present invention protects the camera from this type of damage.
The disclosed invention provides a shutterless calibration solution for in-flight thermal imaging non-uniformity correction that is robust, reliable, and lightweight and does not have the reliability issues associated with an electro-mechanical shutter and also minimizes the risk of imager damage due to direct viewing of the sun.
The present invention is gravity-operated and configured in a UAS such that the imager lens is obscured from view whenever it points above the horizon. In-flight calibration can be initiated by a calibration command to the camera whenever the lens points above the horizon.
This can be done continuously at predetermined calibration intervals or as a “calibration command” given to the UAS that puts it in a right-turn flight mode (for a camera system looking left). The device may be configured wherein a right turn automatically initiates calibration depending on the user's design preference.